As is known in the art, conventional maritime radars for shipboard navigation and for coastal surveillance (e.g., land-based Vessel Tracking Systems (VTS)) detect and measure the position of ships and other discrete contacts in a two-dimensional space. This space is typically idealized as a local-level (tangent) plane to the Earth coordinatized in natural radar polar coordinates of range R and true azimuth A or in cartesian coordinates (x,y) with positive x pointing in an Easterly direction and positive y pointing North. When these radars are also configured with Automatic Radar Plotting Aids (ARPA), the position of a contact is tracked over time, and a velocity vector is derived.
Shipboard ARPA radars are used to automatically alert the ship's watch of collision potentials by automatically detecting and tracking other ships and generating an alert under certain conditions involving the relative motion of ownship and the contact under track by the radar. By assuming both ownship and the contact under track will keep their velocities constant, it is possible to compute a Closest Point of Approach (CPA) and a Time until Closest Point of Approach (TCPA). If both CPA and TCPA are smaller than selected limits, then a collision potential alert is issued.